1. Field of the Invention
The present invention relates to a control method and apparatus of an ultrasonic motor, and an ultrasonic motor driver and controller, especially to a control method and apparatus of an ultrasonic motor, and an ultrasonic motor driver and controller enabling to control accurately the displacement and/or the velocity of a driven object driven by the ultrasonic motor.
2. Description of the Related Art
An ultrasonic motor consists of a piezoelectric element, which makes elastic movement when a voltage is applied across the piezoelectric element, and an elastic body, which transfers the elastic motion of the piezoelectric element to a driven object driven by the ultrasonic motor.
The ultrasonic motor drives the driven object linearly, by applying an alternative voltage to vibrate the piezoelectric element with the elastic body contacting to the driven object.
When the ultrasonic motor is applied to drive a stage of a microscope, it is necessary to control accurately the displacement and the velocity of the stage in order to put an inspecting object in perspective of the microscope.
Up until now, the velocity of the driven object was controlled by a voltage, a frequency, and a phase of the electric power applied to the ultrasonic motor, and the displacement of the driven object was controlled by the number of the driving waves applied to the ultrasonic motor (See Japanese unexamined patent publication No. 11-150962).
The above-mentioned control method, however, cannot accurately control the displacement and/or the velocity of the driven object, because of the characteristic difference between individual ultrasonic motors and the difference of contacting status of the ultrasonic motor to the driven object.
Therefore, various control methods and apparatuses of the ultrasonic motor have been proposed to improve control accuracy.
For example, there has been proposed the apparatus to control the velocity of the driven object by making a means to change the frequency of the driving waves supplied to the ultrasonic motor active while the amplitude of the driving waves is kept at constant, and a means to change the amplitude active while the frequency is kept at constant (See Japanese Patent No. 3220932)
It is difficult, however, to dissolve the following problems with the above-mentioned apparatus.
1. The ultrasonic motor cannot accurately control the displacement of the driven object, when driving waves are continuously supplied, because the friction force changes considerably at the beginning and ending of the displacement of the driven object.
2, The relationship between the change of the frequency of the driving waves and the change of the velocity of the driven object is not linear but nonlinear, and the nonlinear characteristic cannot be explicitly understood.
3. The velocity of the driven object becomes faster as the amplitude of the driving waves is larger, but the displacement resolution is deteriorated concurrently because the displacement of the driven object per one driving wave becomes large.